1. Technical Field
This invention relates to an aqueous coating composition such as coatings, inks, overprint, varnish and surface treatment applications containing an oxazoline or oxazine methacrylate polymer.
2. Background Art
An important consideration in the coating industry today is the reduction of volatile organic compounds (VOC's) in commercial paint and coatings systems. For waterborne systems this means a lower coalescent solvent level, which in turn necessitates the use of polymers with lower glass transitions (Tg's) in order for film formation to take place. Use of such polymers often results in films with inferior properties. In order to counteract this effect the Tg of the film can be raised after it has been applied. One of the most logical ways to accomplish this is by crosslinking. Traditionally, films have been cured by heating at relatively high temperatures for given periods of time. In many applications such heating is not feasible, which means polymers containing functionalities reactive enough to cure at ambient temperature must be synthesized. For waterborne polymers such functionalities must also have reasonable hydrolytic stability. Also, it is desirable if the coating materials can be polymerizable via free radical initiation. Accordingly any monomer candidate must have suitable unsaturation in addition to a curable moiety.
Monomers that meet all of these criteria are few in number, but they do exist; e.g. glycidyl (meth) acrylate, 3-isopropenyl-.alpha.,.alpha.-dimethylbenzyl isocyante, and acetoacetoxyethyl methacrylate are all currently in use in waterborne systems. There is a need for an improved monomer which will meet these criteria such as oxazoline or oxazine mechacrylate monomers which can be incorporated as polymers in an aqueous coating composition.
Oxazoline and oxazine substituted acrylic esters are described in U.S. Pat. No. 3,996,237 for use in coatings. These acrylic esters were produced by the reaction of bis-oxazoline or oxazine with an acrylic acid. In this reaction the carboxylic acid reacts with presumably one of the oxazoline rings to produce an acrylic monomer containing oxazoline moiety. However, this type of synthetic method is well known to give a mixture of di-acrylate, unreacted bis-oxazoline in addition to the desired monomer. In addition this approach is limited to the ester-amide link.
U.S. Pat. No. 4,247,671 to Reitz et al. discloses an oxazoline containing acrylic monomer specific for use in a hydrocured thermoset polymer with carboxylic cyclic anhydriedes. These oxazoline acrylic monomers contain aliphatic substitution in the 2-position of the oxazoline ring. This type of substitution, we discovered, is susceptible to hydrolysis in water and consequently can not be used in the aqueous system.
In addition, reactive polycarbonates end capped with hydroxy phenyl oxazoline are described in U.S. Pat. No. 4,940,771 and aliphatic substituted oxazolines are described in U.S. Pat. No. 3,464,933.
Other types of 2-oxazoline containing polymers are described in U.S. Pat. No. 5,300,602. These polymers are based on 2-isopropenyl-2-oxazoline which is functionally different than the monomer in our invention. In addition 2-isopropenyl-2-oxazoline has a limited use.